Many times process fluids that are flowing through a valve collect in internal cavities in the valve. A typical rotary valve, for example, includes a valving member that is disposed in a fluid passage of a valve intermediate the valve's inlet and outlet. This valving member is rotated about an axis that is generally perpendicular to the fluid flow path through the valve, and the valving member is rotated to selectively bring an internal passageway that extends through the valving member into and out of registry with the valve's fluid flow path. When the valving member of a two-way valve is in its fully open position, its internal passageway is aligned and becomes a part of the flow path through the valve. Such a two-way valving member is movable to the fully closed or shutoff position by rotating the valving member 90 degrees from its fully open position. When so rotated, the internal fluid passageway through the valving member no longer communicates with the fluid flow passage through the valve, and the valving member functions to block fluid flow through the valve.
When the valving member is moved from open to closed positions while a process fluid is flowing through the valve, a small amount of the process fluid is trapped in the valving member's internal fluid passageway, sealed from both the valve inlet and valve outlet. While only a relatively small amount of process fluid is trapped, such trapped fluid has proved to be problematic when the valve is exposed to extremely high temperatures and pressures, such as occurs during fires. Under such extreme conditions, the trapped fluid within the valving member's fluid passageway becomes superheated, and the pressure and temperature within the closed cavity in which the fluid is trapped may be greater than the design capabilities of the valve, resulting in structural failure.
Recognizing the problems of trapping fluid in valve cavities, several prior art attempts have been made to alleviate such problems by venting such trapped fluid. In one approach, excess pressure in internal valve cavities is relieved by self relieving valve seals that are designed to rupture or otherwise relieve pressure at predetermined temperatures and pressures. Other valve designs rely upon the valve seals themselves to decompose sufficiently during a fire to relieve any excess pressure. However, in most cases, excess pressure is reached before the seals have reached a guaranteed decomposition temperature.
In another approach, vent holes are formed in the valve components to insure that the internal cavities are always in fluid communication with another location of the valve to which excess pressure can be relieved. A common cavity vented in the prior art is the internal passageway of a rotatable valving member. For example, the cavity formed by the valving member's internal flow passageway sometimes is vented by vent holes that extend between the valving member's internal passageway and a location external to the valving member. These vent holes are usually drilled in a direction perpendicular to the direction of the valving member's internal flow passage, either through the side wall (to the internal flow passage of the valve body) or the bottom wall (to the closed portion of the valve chamber opposite the shaft opening) of the valving member.
While the venting of valve cavities has proved advantageous, it is not without difficulties. For example, if the internal flow passage of a plug valve or a ball valve is vented, it must be vented on the upstream side of the valving member. Otherwise, any potential leakage past the primary seals on the upstream side of the valving member (which leakage is rendered more probable due to possible downstream shifting of the valving member relative to the valve body from line pressure) will flow around the valving member, through the valving member's internal flow passage, and leak through the vent hole. As a consequence, prior art valves with venting of a valving member's internal flow cavities generally have been rendered uni-directional. Of course, unidirectional valves must be marked to indicate the required flow direction for the valve. Further, unidirectional cannot be used in all applications, and inherently add increased risk for malfunction due to improper assembly or installation.